I don't want this thread to get locked, since it's such a worthy one, so I hope everyone is careful with what you mention about your ritual.
I think I'm a good example of a non-Christian who has had no problems in a sorority that was founded heavily on Christian traditions. That doesn't mean that the sorority is trying to convert me. Our rituals, symbols and traditions also draw from Egyptian lore, Hindu mysticism, and Greek mythology -- but that doesn't mean that we're supposed to convert to any of those beliefs. Almost all sororities draw at least in part from some sort of Greek history or mythology, but that doesn't mean they're advocating conversion to that!
You can try thinking of the references to Jesus or God or whatever it is that you don't believe in as symbols -- they're just using them to better illustrate the concepts of love, sisterhood, loyalty, service, etc. Of course I don't know any sorority's ritual but my own, but I'd be surprised if any of them insisted you believe in Jesus or God in order to join. All that they ask is that you believe in the concepts that Jesus/God/whatever is illustrating -- sisterhood, loyalty, service, love.